Code N is for a recharacterization, not a Roth conversion. A recharacterization is not taxable as income, but might have resulted in an excess Roth IRA contribution that is subject to a 10% excess contribution penalty depending on your MAGI and filing status.
A code-N Form 1099-R is required to have a zero in box 2a because the transaction is always nontaxable. TurboTax knows that a code-N distribution is nontaxable and will properly ignore anything you put in box 2a.
